This page collects links for developers and groups them by the need that brings you here. The tool group gathers web utilities. HTTPie handles API testing, Typst handles typesetting, and other entries cover JavaScript obfuscation, JSON editing, cron scheduling, spreadsheet conversion, and database ERD diagrams. The algorithm group points to practice and study sites. HackerRank, LeetCode, SPOJ, Codeforces, and Codewars host problem sets and contests. VNOI Wiki, VisuAlgo, MIT OpenCourseWare, and several Vietnamese sites supply tutorials, visualizations, and structured exercise plans. Every entry carries a short description of what the site offers, so you can skim the list and open only the few that fit your task.
- tool
- HTTPie for Web is a browser-based API testing client that offers a clean, intuitive interface for sending HTTP requests and inspecting responses. Built as the web version of the popular HTTPie command-line tool, it supports all HTTP methods, custom headers, authentication, request bodies, and query parameters. Users can organize requests into collections, save and share them, and view responses formatted with syntax highlighting. The app is designed to make API testing and debugging easy, a modern alternative to Postman with a focus on simplicity.
- Typst is a modern markup-based typesetting system and collaborative web app, an alternative to LaTeX and Word. It lets you write documents in a clean, intuitive markup language with real-time preview and instant compilation. It supports math, bibliographies, images, tables, code highlighting, and presentations out of the box, with a powerful embedded scripting language for automation. The platform offers team collaboration, private packages, Git integration, templates, and Zotero sync. With more than 1,200 community packages and templates, Typst is used for papers, theses, and reports.
- aaencode is an online tool that converts JavaScript programs into code that uses only Japanese-style kaomoji. Users enter standard JavaScript and the tool encodes it into code made up solely of emoticons such as ^_^ that can still be executed. The encoded output can be run with JavaScript eval and produces the same result as the original code. The tool was made mainly for fun and to demonstrate JavaScript encoding techniques, showing the flexibility of character encoding in the language.
- Obfuscator.io is a powerful online JavaScript obfuscation tool that protects JavaScript source code from reverse engineering and theft. It transforms readable code into hard-to-understand code through string encoding, control flow flattening, dead code injection, and self-defending mechanisms. The platform also offers advanced VM obfuscation that turns JavaScript into custom bytecode running on an embedded virtual machine, making deobfuscation extremely difficult. It supports many configuration presets, Node.js optimization, and provides both a free playground and paid VM-protection plans.
- JSON Editor Online is a versatile web-based tool for viewing, editing, formatting, and processing JSON data. It offers several editing modes, including text, tree, and table, making it easy to navigate and modify complex JSON structures. Key features include JSON formatting, schema validation, document comparison, JSON repair, data querying and transformation, and CSV conversion. The editor supports files up to 512MB, works offline, and lets you save and share documents, making it an essential tool for frontend and backend developers.
- Cron-job.org is a free online cron job scheduling service that executes website URLs and scripts at configurable intervals from every minute to every year. Key features include execution history with response data, custom HTTP request configuration, test runs, status notifications when a job fails or recovers, status pages and monitoring badges, multi-factor authentication, and a REST API for programmatic management. The service runs on CO2-neutral hydroelectric power and has been operating for more than 15 years, serving hundreds of thousands of users.
- Mr. Data Converter is a free browser-based tool that converts spreadsheet data from Excel and CSV into web-friendly formats including HTML, JSON, and XML. Users simply paste their data and choose the output format. It supports delimiter configuration, including auto-detect, comma, and tab, as well as decimal separator options. Additional settings let you treat the first row as a header, apply text transformations such as lowercase or uppercase, and add whitespace indentation to the output.
- erd-editor is an open-source Entity-Relationship Diagram editor available as a web app, a VSCode extension, and an IntelliJ plugin. It supports PWA for offline use, real-time collaboration between multiple users, end-to-end encryption for secure sharing, and a local-first architecture with browser autosave and real-time sync across tabs. The tool lets developers visually design database schemas, define tables, columns, and relationships, and generate SQL statements. Built with TypeScript, MIT license.
- DrawDB is a free online database diagram editor and SQL generator for visually designing database schemas through an intuitive drag-and-drop interface. Users can create tables, define columns with data types, set primary and foreign keys, establish relationships between tables, and then automatically generate SQL scripts for multiple database systems. It removes the need to install desktop software by running entirely in the browser, a convenient solution for developers, DBAs, and students.
- algorithm
- HackerRank is an online platform for practicing and assessing technical programming skills across many domains including algorithms, data structures, mathematics, artificial intelligence, and databases. It supports many programming languages such as C, C++, Java, Python, Ruby, and SQL. The platform offers AI-powered mock interviews covering technical screens, coding challenges, system design, and behavioral questions. Users can prepare for job interviews, earn certifications, and compete in coding contests. Companies also use HackerRank to recruit and assess candidates' skills.
- LeetCode is a popular online platform for practicing coding problems and preparing for technical interviews. It offers thousands of algorithm, data structure, and system design problems from easy to hard. Users can solve problems in many programming languages, submit for instant evaluation, and discuss approaches with a large community. The platform provides study plans, mock assessments, contests, and company-specific question sets. It is widely used by software engineers and job seekers to sharpen problem-solving skills and prepare effectively for coding interviews.
- SPOJ (Sphere Online Judge) is an online judge platform hosting a large collection of programming problems across many categories and difficulty levels. The classical problem set contains thousands of algorithmic challenges covering dynamic programming, graph theory, number theory, string processing, and more. Users can submit solutions in more than 45 programming languages and get immediate feedback on correctness and efficiency. The platform supports competitive programming practice with rankings, contests, and a global community, and is widely used to improve algorithmic thinking.
- Codewars is an online coding practice platform where developers improve their skills by solving community-created coding challenges called kata in more than 55 programming languages. The platform follows a martial-arts ranking system, with users advancing from beginner to expert by completing increasingly difficult kata. Key features include test-driven development with instant feedback, the ability to compare solutions with others, community kata creation, mentoring opportunities, and competitive leaderboards. Codewars emphasizes learning by doing.
- Codeforces is a competitive programming platform that regularly hosts ranked algorithm contests for programmers of all levels. The site runs rounds divided into divisions based on participant rating, with problems designed by community authors. It offers a huge problemset for practice, gym training sessions, educational courses, team contests, and a comprehensive rating system that tracks performance. It supports many programming languages and has an active discussion community. Codeforces is one of the most popular competitive programming platforms worldwide.
- 300 Bài Code Thiếu Nhi (300 Kids' Code Problems) is a Vietnamese-language website collecting 300 programming exercises based on the well-known Grind 75 set. The problems are split into easy, medium, and hard levels and cover many topics such as Array, Binary Tree, Dynamic Programming, Graph, Linked List, Stack, and String. Exercises are arranged by study week, and each has a note with its estimated completion time. It is a useful practice resource for learners who want to prepare for technical interviews, train algorithmic thinking, and improve problem-solving skills.
- VNOI Wiki is the wiki of the Vietnamese competitive programming community, offering comprehensive knowledge of algorithms and data structures. It includes detailed tutorial articles on advanced algorithms such as dynamic programming, graphs, number theory, computational geometry, and many other topics. It is a valuable study resource for Vietnamese students training in competitive programming, preparing for the Informatics Olympiad, or wanting to improve their algorithm skills. Content is contributed by the community and continuously updated.
- MIT OpenCourseWare provides free lecture notes from the Introduction to Algorithms (6.006) course taught by Prof. Erik Demaine and Srini Devadas. The course covers fundamental algorithm topics organized into 8 units: sorting and trees, hashing, numerical algorithms, graph algorithms, shortest paths, dynamic programming, and computational complexity. Each unit has detailed PDF lecture notes, handwritten originals, and example Python code. An invaluable undergraduate-level resource for computer science students who want to learn algorithms from one of the world's leading universities.
- Cowboy Coder Tech is a website sharing knowledge and resources on programming, software engineering, and development skills for programmers. The site offers articles, tutorials, and practical experience to help developers improve their coding ability and build products. It is a useful reference for anyone who wants to improve their programming skills and pursue a career in information technology.
- A GitHub repository collecting detailed solutions to CTF (Capture The Flag) challenges, mainly in cryptography. It includes writeups from many well-known security competitions such as Google CTF, TetCTF, AceBear Security Contest, SVATTT, and Tokyo Western CTF. The author also shares a cryptography learning path from basic to advanced, with material on algebra, probability and statistics, algorithms, and cryptography. Very useful for anyone studying information security and cryptanalysis.
- Codewars is an interactive coding challenge platform where developers improve their skills by solving kata, small coding exercises organized by difficulty. It supports more than 55 programming languages including CSharp, Python, and JavaScript. Users earn rank and honor points as they progress, creating a gamified learning experience. Each kata is created by the community and peer-reviewed. Codewars is ideal for programmers of all levels who want to hone problem-solving skills, learn new languages, and compare solutions with a global community.
- Giải Thuật Lập Trình (Programming Algorithms) is a Vietnamese website dedicated to materials and lectures on algorithms, data structures, and competitive programming. It covers topics from basic to advanced such as sorting, searching, dynamic programming, graphs, trees, and many classic algorithms. It is a helpful study resource for Vietnamese students learning data structures and algorithms or practicing programming on their own. Vietnamese-language content makes it easy for learners to absorb.
- VisuAlgo is an interactive web-based visualization tool developed by the National University of Singapore that shows data structures and algorithms through step-by-step animation. It has 24 visualization modules covering sorting, graph traversal, shortest paths, minimum spanning trees, dynamic programming, network flow, and computational geometry. Users can enter custom data, and the platform includes an online quiz system. Available in English, Chinese, and Indonesian, VisuAlgo is an excellent educational resource for computer science students and teachers.
- The data structures overview page on VNOI Wiki presents foundational and advanced knowledge of data structures used in competitive programming. Content covers arrays, linked lists, stacks, queues, trees, graphs, hash tables, segment trees, Fenwick trees, and many other special structures. Each structure comes with basic operations, complexity, and practice problems. It is an essential reference for Vietnamese students preparing for competitive programming who want to master data structure fundamentals.
- Topcoder is a platform that combines competitive programming with enterprise crowdsourced talent solutions. Originally famous for its algorithm programming contests, Topcoder has grown into a talent marketplace connecting businesses with a global network of vetted developers, designers, and data scientists. The platform offers crowdsourced innovation, rapid prototyping, application development, and quality assurance. It serves many industries including healthcare, energy, finance, and technology, and is useful for both businesses and programmers.
- CodeLearn is a Vietnamese online coding-learning platform offering courses and hands-on exercises on many programming languages and technologies. It supports learning through interactive exercises, challenges, and coding tracks that build practical coding skills. It suits beginners through experienced programmers who want to level up. The platform includes guided learning, algorithm practice, and interview preparation, helping to develop a programming career effectively.
- Free Contest is an online competitive programming platform for Vietnamese students. It offers regular programming contests at difficulty levels from basic to advanced. Users practice algorithms, join weekly contests, and follow the leaderboard. It is an ideal place to build programming skills and prepare for the Informatics Olympiad and technical interviews, and it supports many popular programming languages. @@CONTINUE@@